1. Field of the Invention
This invention relates to a capped wheel nut having a nut body with a conical end and a stainless steel sheath covering the wrench flats and the end opposite to the conical end, and terminating adjacent the joinder between the wrench flats and the conical end, and to a method and apparatus for resistance welding the sheath to the nut body.
2. Prior Art
Most modern automotive wheels are removably attached to the axle by a ring of threaded studs which extend outwardly from conical depressions formed in the axle end. The wheel contains a similar ring of holes formed with conically dished edges which abut the depressions in the axle end so that the studs extend through the wheel holes. Wheel nuts are then threaded on the studs to secure the wheel to the axle. The nuts and extending studs may be surrounded by a dish-shaped cover which attaches to the wheel to hide and protect the nuts and studs. Alternatively, wheel nuts having integral caps that enclose the volume about the end of the nut opposite to that which contacts the wheel may be employed instead of the wheel covers. The cap protects the projecting end of the stud from damage due to weathering and impacts.
These capped nuts have typically been formed out of bar stock, on turning machines, with the caps being formed integrally with the nut body. Alternatively, capped nuts have been proposed wherein a conventional nut body is fully covered by a stainless steel sheath which exposes one threaded opening of the nut and forms a domed cover over the other opening. U.S. Pat. No. 3,364,806, assigned to the assignee of the present invention, discloses a variation on the sheathed wheel nut wherein the conical end section of the nut body, which is adapted to bear against a conical depression in a wheel, is left unsheathed, with the sheath terminating at a land formed between one end of the wrench flats and the adjacent section of the conical end.
On this nut the free edges of the sheath are crimped against a shoulder formed at this land to retain the sheath to the cap but wrenching forces and accidental road impacts from stones and the like may loosen the sheath so that it tends to rattle when the vehicle is driven. Adhesives have been applied between the contacting surfaces of the sheath and body to prevent this loosening, but it is difficult and expensive to apply an amount of adhesive which will securely retain the sheath to the cap without getting any adhesive on the threads.
It has also been proposed to weld the cap to the nut. This insures rigid attachment and eliminates the danger of clogging the nut threads with the adhesive, but presents certain problems in application. For one thing, the heat generated in the welding process may adversely affect the heat treatment of the nut, as by unacceptably softening it, and may affect the metallurgy of the cap to discolor it or reduce its oxidation resistance.
Welding systems which pass a low voltage, high current pulse of very short duration through the interface between the pair of metallic members have previously been employed in certain applications wherein at least one of the metallic elements would be damaged by the passage of the more conventional lower current, longer duration resistance welding pulses. These systems typically employ relatively large capacitors to accumulate the welding power from a direct current power supply. After the capacitors are fully charged a switch is closed which passes the capacitor current through the two parts to be welded.
The present invention is broadly directed toward a method of welding the stainless steel sheath of a capped wheel nut to the nut body, using capacitive discharge welding, to form positive engagement between the two which will resist the forces that are applied during wrenching of the nut and by virtue of accidental impacts during use, to prevent loosening of the nut and the resultant rattling noises produced when a vehicle with a loosened cap is driven.